The present invention is directed to polyaxial bone screws for use in bone surgery, particularly spinal surgery and particularly to such screws with compression or pressure inserts and expansion lock split retainers to snap over, capture and retain the bone screw shank head in the receiver member assembly and later fix the bone screw shank with respect to the receiver assembly.
Bone screws are utilized in many types of spinal surgery in order to secure various implants to vertebrae along the spinal column for the purpose of stabilizing and/or adjusting spinal alignment. Although both closed-ended and open-ended bone screws are known, open-ended screws are particularly well suited for connections to rods and connector arms, because such rods or arms do not need to be passed through a closed bore, but rather can be laid or urged into an open channel within a receiver or head of such a screw. Generally, the screws must be inserted into the bone as an integral unit along with the head, or as a preassembled unit in the form of a shank and pivotal receiver, such as a polyaxial bone screw assembly.
Typical open-ended bone screws include a threaded shank with a pair of parallel projecting branches or arms which form a yoke with a U-shaped slot or channel to receive a rod. Hooks and other types of connectors, as are used in spinal fixation techniques, may also include similar open ends for receiving rods or portions of other fixation and stabilization structure.
A common approach for providing vertebral column support is to implant bone screws into certain bones which then in turn support a longitudinal structure such as a rod, or are supported by such a rod. Bone screws of this type may have a fixed head or receiver relative to a shank thereof, or may be of a polyaxial screw nature. In the fixed bone screws, the rod receiver head cannot be moved relative to the shank and the rod must be favorably positioned in order for it to be placed within the receiver head. This is sometimes very difficult or impossible to do. Therefore, polyaxial bone screws are commonly preferred. Open-ended polyaxial bone screws typically allow for a loose or floppy rotation of the head or receiver about the shank until a desired rotational position of the receiver is achieved by fixing such position relative to the shank during a final stage of a medical procedure when a rod or other longitudinal connecting member is inserted into the receiver, followed by a locking screw or other closure. This floppy feature can be, in some cases, undesirable, but may not be that detrimental in others. Also, it is often desirable to insert the bone screw shank separate from the receiver or head due to its bulk which can get in the way of what the surgeon needs to do. Such screws that allow for this capability are sometimes referred to as modular polyaxial screws.